The Xaudia Blog

08-06-2010

Thiele microphone brochure

Thiele M5 tube microphone
Here’s a scan of a short advertising document for Thiele M4 and M5 (photo to the left) tube microphones. 

Theile sales document
Note how expensive the microphones were at the time – 500 and 600 Deutchmarks. For reference, between 1950 and 1960, 4 Deutchmarks approximately equalled 1 US dollar.
07-06-2010

Syncron AU7A microphones Part 2

Last time I wrote about a pair of Syncron AU7A microphones. The capsules were in good condition, but the batteries had leaked, causing corrosion and damage to the circuit. For one of these mics I decided to fit a tube circuit based on a 6205 subminiature tube (5840* would do just as well or better)**.

Tube modified Syncron microphone circuit, 6V regulated heater supply omitted.

The Syncron capsule operates happily between about 40V and 60V, and a simple voltage divider was used to supply the backplate with a suitable polarising voltage. As the capsule is cardioid only, the circuit can be made as simple as possible, and there is no need for a capacitor between the diaphragm and the tube grid.

With a little creative hacking I was able to reuse the circuit board to construct a valve circuit, which avoids damaging the microphone further. Although physically larger, the tube sits where the transistor was (I even used the same PCB pads as the FET), and there is room on the underside of the board for a couple of capacitors. An added bonus is that the original transformer is quite suitable for use in a tube circuit, and was rewired in 10:1 configuration. The rest of the circuit – 5 resistors and another cap – fit on the ‘wrong’ side of the board in the cavity below the capsule housing. Then it is a simple case of wiring the connector to the circuit and connecting the capsule, taking care not to damage the diaphragm.

One thing to look out for with this arrangement is that the amphenol cable plug & connector on the microphone are the reverse of the normal gender, which means that there can be 110V DC on the exposed pins. Consequently care must be taken to connect the microphone before the power supply is turned on, otherwise a short sharp shock can happen. Of course this isn’t really an acceptable acceptable solution from a safety point of view.
In practice the microphone works very nicely and is suitably quiet for recording vocals. We tracked some female vocals with it yesterday and it performed very well in that application.
Meanwhile, I have managed to track down some 22V batteries from Farnell, which should be suitable for the capsule polarisation, so I’ll attempt to restore the second mic to its original state. More on that soon.
** With hindsight the 5840 may be a better bet as there is an internal connection between the cathode and grid 3. This allows you to cut off two of the leads, which means using up one less precious pad on the circuit board inside – space is tight!
**Readers familiar with the ‘Royer’ tube circuit will recognise the topology, although a few of the component values are different.

Xaudia blog post on phantom power for these mics.

06-06-2010

Syncron AU7A (Fairchild F/22) microphones Part 1

I was lucky enough to come across a pair of Syncron AU7A microphones (aka Fairchild F/22) on ebay. On arrival from the US I found that all the foam lining in the boxes had decomposed and spread black dust everywhere. Luckily the capsules appeared to be in fine condition, and the mics came with the original cables, so the should be a good chance of getting them back to working condition.

That’s easier said than done! The mics run on 4 batteries – 2 x 4.2V for the amplifier and 2 x 21V for the polarisation. Unfortunately, our microphones came complete with the original vintage batteries inside, which had inevitably leaked and caused corrosion throughout. The batteries are now pretty much unobtainable, so I used a bench voltage supply to simulate the batteries. Microphone number one gave a very weak and noisy (hiss) signal – I suspected the FET had somehow become contaminated by the battery acids. Mic 2 was slightly better, but certainly not something you could use as a serious recording tool.
These are reported to be the first commercially available FET microphone, and searching the internet didn’t throw up any schematics so I traced out the circuit, which is very very simple – capsule -> field effect transistor -> DC blocking cap -> transformer.

EDIT 21/9/2011 : please note that the schematic posted here contained errors. A revised version is here!

The transformer may be wired either for 200 or 50 ohms, and measurement showed it has a voltage ratio of 5:1 in series or 10:1 in parallel mode.
At this stage I needed to make a decision on how to get the best out of the microphone. More on that very soon, but for now here are some web links to Syncron information – there’s not a lot of it about!
16-02-2010

Neumann Gefell UM57 experiments

Lately I’ve had the opportunity to play around with several vintage Neumann Gefell tube microphones – a CMV563 (below with UM70 capsule) , a M582 and a pair of UM57s.

These all have broadly similar circuits, with a EC92 tube and transformer coupled feedback. The UM57 is configured for different polar patterns, whereas with the CMV563 and M582 you have to swap the capsule. There are other differences – the schematics are shown here.

One particularly common fault with examples of these microphones is that the original electrolytic output capacitor can dry out with age. This is by no means always the case, and the capacitor in the UM57 on the left above was in perfect condition after nearly 50 years!  The one on the right has been replaced with an orange modern metalised film capacitor.

So what is the effect of ageing of this capacitor? As the electrolyte dries out, the absolute value of the capacitance drops, which will affect the frequency response of the valve amplifier inside the microphone. To simulate this, a capacitance decade box was wired in place of the output capacitor (C3), and the chart below shows how the frequency response changes as the capacitance decreases in 0.2 uF steps.*

Part of the circuit is shown inset within the chart. Although intuitively we expect the smaller capacitor to give us less bottom end, the network of the capacitor, transformer primary winding and resistor acts as a resonant filter, producing a peak in the bass region just above a sharp drop off. The human ear can perceive this as more bass – although not necessarily in a good way: the microphone may seem muddy or lack clarity.

So, having a good quality capacitor here is vital, and the value of this can be used to tweak the bass response if desired. Of course this analysis is just for the tube circuit inside the mic and does not consider the effects of ageing on the capsule itself – that’s a story for another day.

SJT Feb 2010

* Measured using a swept-sine wave from 1Hz to 48KHz.

12-02-2010

Recording with the Josephson C720

Back in August we were lucky enough to get our hands on a special edition Josephson C720 microphone for the studio, one of only 20 made in the first production run, and having lived with this for nearly half a year it’s probably about time we reviewed it.
josephson C720 microphone
The first thing you notice about the C720 is its size – this is one big mic, U47-big, and makes a big bold statement in the live room or vocal booth – it’s built like a tank and it definitely has that ‘wow’ factor which gets the artists talking.
The next thing that grabs your attention is its radical aluminium metal-foam headbasket, which looks like something you could scrub your pots and pans with. The non-periodic headbasket is actually designed to eliminate internal acoustic reflections and refraction of soundwaves, avoiding comb-filtering effects and so giving a more realistic recording.
The third thing you notice is that it has two XLR outputs. Like many condenser microphones, the capsule has dual diaphragms which can be combined in different combinations to give cardioid, omni, figure of eight patterns. Unlike most microphones, the two transducers have separate head amplifiers and separate outputs which means that the signals from opposite sides of the microphone can be recorded separately and mixed together, either in phase or with the polarity reversed, at a later stage. What this innovation means in practice is that you can pick the polar pattern after recording, which might be used to eliminate unwanted sound sources by rotating the null point of the microphone. Also, and more importantly in our studio use, with close-mic’ed sources such as vocals you can dial in more or less proximity effect, making a singer seem bigger, darker or lighter and brighter.
So what does it sound like? Possibly because of the lack of head basket comb filtering, and the facility to tune the pattern and proximity effect, this is a very versatile microphone. The one word that sprang to my mind in describing the sound, whichever pattern you pick, is ‘solid’.*
Another thing we’ve noticed is it sounds pretty damned good on pretty much anything you can throw at it. Because of it’s ability to handle high sound pressure levels and become a go-to mic for low end stuff, and is rock-solid on bass guitar and front of kick drum, but it’s just as handy on both male and female vocals – especially those with a tendency to get loud when they belt it out. Vocal recordings are up front, sound as they should, but without the top end brightness of, say, our TLM49.
Furthermore, it can be used for more radical effects by compressing or reverberating one side of the capsule only. For example, I had some interesting results on a male rock vocal by compressing the signal from the front capsule, mixing together with the phase reversed rear capsule, then compressing the sum of these. The effect was that as the vocals push harder, the relative amount of rear capsule included becomes greater giving it more of a hypercardioid pattern, balancing the tendency of the vocalist to step back from the mic when belting it out. With a bit of creativity the possibilities are endless.
Overall this is a brilliant, radical piece of thinking and and one of those bits of gear that comes into your life and is there to stay.
*After writing this I read another review of this mic which used exactly the same word – ‘solid’ – to sum up the sound.

07-01-2010

New Toy!

AVO MkIII tube testerHere is our new pride and joy – a 1960’s AVO mkIII valve characteristic meter. It’s built like a tank, with wonderful vintage knobs and dials. And comes in handy for getting those amps and mics up and running.